Source: https://zod.dev/error-formatting?id=zflattenerror

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Formatting errors

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Zod emphasizes completeness and correctness in its error reporting. In many cases, it's helpful to convert the $ZodError to a more useful format. Zod provides some utilities for this.

Consider this simple object schema.

import * as z from "zod";
 
const schema = z.strictObject({
  username: z.string(),
  favoriteNumbers: z.array(z.number()),
});

Attempting to parse this invalid data results in an error containing three issues.

const result = schema.safeParse({
  username: 1234,
  favoriteNumbers: [1234, "4567"],
  extraKey: 1234,
});
 
result.error!.issues;
[\
  {\
    expected: 'string',\
    code: 'invalid_type',\
    path: [ 'username' ],\
    message: 'Invalid input: expected string, received number'\
  },\
  {\
    expected: 'number',\
    code: 'invalid_type',\
    path: [ 'favoriteNumbers', 1 ],\
    message: 'Invalid input: expected number, received string'\
  },\
  {\
    code: 'unrecognized_keys',\
    keys: [ 'extraKey' ],\
    path: [],\
    message: 'Unrecognized key: "extraKey"'\
  }\
];

z.treeifyError()

---

To convert ("treeify") this error into a nested object, use z.treeifyError().

const tree = z.treeifyError(result.error);
 
// =>
{
  errors: [ 'Unrecognized key: "extraKey"' ],
  properties: {
    username: { errors: [ 'Invalid input: expected string, received number' ] },
    favoriteNumbers: {
      errors: [],
      items: [\
        undefined,\
        {\
          errors: [ 'Invalid input: expected number, received string' ]\
        }\
      ]
    }
  }
}

The result is a nested structure that mirrors the schema itself. You can easily access the errors that occurred at a particular path. The errors field contains the error messages at a given path, and the special properties properties and items let you traverse deeper into the tree.

tree.properties?.username?.errors;
// => ["Invalid input: expected string, received number"]
 
tree.properties?.favoriteNumbers?.items?.[1]?.errors;
// => ["Invalid input: expected number, received string"];

Be sure to use optional chaining (?.) to avoid errors when accessing nested properties.

z.prettifyError()

---

The z.prettifyError() provides a human-readable string representation of the error.

const pretty = z.prettifyError(result.error);

This returns the following string:

✖ Unrecognized key: "extraKey"
✖ Invalid input: expected string, received number
  → at username
✖ Invalid input: expected number, received string
  → at favoriteNumbers[1]

z.formatError()

---

This has been deprecated in favor of z.treeifyError().

Show docs

z.flattenError()

---

While z.treeifyError() is useful for traversing a potentially complex nested structure, the majority of schemas are flat—just one level deep. In this case, use z.flattenError() to retrieve a clean, shallow error object.

const flattened = z.flattenError(result.error);
// { errors: string[], properties: { [key: string]: string[] } }
 
{
  formErrors: [ 'Unrecognized key: "extraKey"' ],
  fieldErrors: {
    username: [ 'Invalid input: expected string, received number' ],
    favoriteNumbers: [ 'Invalid input: expected number, received string' ]
  }
}

The formErrors array contains any top-level errors (where path is []). The fieldErrors object provides an array of errors for each field in the schema.

flattened.fieldErrors.username; // => [ 'Invalid input: expected string, received number' ]
flattened.fieldErrors.favoriteNumbers; // => [ 'Invalid input: expected number, received string' ]

Customizing errors

Guide to customizing validation error messages and error handling patterns Metadata and registries

Attaching and manipulatinvg metadata on Zod schemas

On this page

z.treeifyError() z.prettifyError() z.formatError() z.flattenError()